During “classical” cell culture in an essentially flat culture vessel, cells in general and biopsies in particular tend to de-differentiate. Visibly, biopsies exhibit the ‘melting ice-cream effect’ as cells migrate from a block of tissue out onto the flat supporting surface of the culture vessel. Gene expression is altered in these “migrating” cells, which begin to behave biochemically as isolated cells rather than as cellular components of a differentiated tissue. De-differentiated cells express different biochemical pathways than those normally expressed by corresponding cells in an intact organism.
In contrast with “classical” cell culture conditions, “microgravity” conditions preserve the differentiation state of many types of cells in culture. Microgravity bioreactors maintain microgravity conditions by continuous rotation of a typically cylindrical or tubular incubation compartment. This rotation continuously forces cells towards the center of the incubation chamber, suspending the cells in a fluid environment using a minimum shear force. This induces them to interact and to aggregate into colonies. For microgravity culturing, cells are often initially sown out onto small (ca. 100 μm diameter) beads (this accelerates the formation of microtissue structures but is not essential). As prototissues are formed by cell growth around these beads, the beads are often ejected or become completely covered with cells. Prototissues formed in this manner become very highly differentiated so as to resemble adult tissue.
Microgravity bioreactors have been used in a variety of contexts. Early studies showed that microgravity bioreactor systems helped cells form three dimensional structures by reducing shear stress on the cells [Reduced shear stress: a major component in the ability of mammalian tissues to form three-dimensional assemblies in simulated microgravity. Goodwin T J, Prewett T L, Wolf D A, Spaulding G F. J Cell Biochem. 1993 March; 51(3):301-11.
Now a significant body of literature demonstrates increased differentiation of cells grown in a microgravity bioreactor system. For a review see: [Growing tissues in microgravity. Unsworth B R, Lelkes P I. Nat Med. 1998 August; 4(8):901-7.] For example, microgravity culturing induces neural precursor cells to form cellular clusters or “neurospheres”. These prototissues are characterized by a crude, but organized, architecture, having a surface layer of immature proliferating cells (nesting-and proliferating cell nuclear antigen-positive) that enclose strata of more differentiated cells (beta-tubulin III-and glial fibrillary acidic protein-positive). These “neurospheres” show promise for development of neurotransplantable tissue. See e.g. [Neural precursor cells form rudimentary tissue-like structures in a rotating-wall vessel bioreactor. Low H P, Savarese T M, Schwartz W J. In vitro Cell Dev Biol Anim. 2001 March; 37(3):141-7.] and see [Rapid differentiation of NT2 cells in Sertoli-NT2 cell tissue constructs grown in the rotating wall bioreactor. Saporta S, Willing A E, Shamekh R, Bickford P, Paredes D, Cameron D F. Brain Res Bull. 2004 December 150; 64(4):347-56].
Or for another example, microgravity culturing of a multipotential human retinal cell line induced expression of a nearly in vivo phenotype, which could not be achieved when the cells were grown under other conditions [Generation of 3D retina-like structures from a human retinal cell line in a NASA bioreactor. Dutt K, Harris-Hooker S, Ellerson D, Layne D, Kumar R, Hunt R. Cell Transplant. 2003; 12(7):717-31.]
Some technical problems with microgravity bioreactors have been reported. For example, when temporomandibular joint (TMJ) disc tissues were engineered using either flat culture or a microgravity bioreactor, there were no significant differences in total matrix content and compressive stiffness, notwithstanding marked differences in gross appearance, histological structure, and distribution of collagen types I and II (with the bioreactor discs having more collagen type II). The authors concluded that improvements of the microgravity bioreactor culture system were needed [Detamore M S, Athanasiou K A. Use of a rotating bioreactor toward tissue engineering the temporomandibular joint disc [Tissue Eng. 2005 July-August; 11(7-8):1188-97].
A significant limitation of microgravity bioreactors of the prior art is moisture loss, which affects cell growth. De-hydration (even only by 5-10%) during incubation can result in changes in pH and other concentration-dependent parameters, such as concentrations of salts, nutrient substances, and the like. Many cell types are highly sensitive to their environment. For such cells, even a small change in such environmental conditions can influence cell growth and gene expression. This problem is especially pronounced in a small volume bioreactor, where small changes in volume can cause relatively large changes in concentration-dependent parameters. Without some solution to this de-hydration problem, a small volume bioreactor would experience rapid loss of moisture, notwithstanding maintenance of humidified conditions (100% relative humidity) in the incubator where the bioreactor was used. This tendency for rapid de-hydration in a small volume bioreactor, that is, this tendency for rapid change in relative volume greatly increases the need for time-consuming manual monitoring and manipulation, for example to replenish or exchange culture medium. This tendency effectively renders long-term maintenance of cultures in a small volume bioreactor impractical or impossible. Accordingly, it would be advantageous to provide a microgravity bioreactor with very high relative water retention in the cell incubation compartment.
WO 07/076865, WO 95/07344, U.S. Pat. Nos. 5,153,131, 5,437,998, 5,665,594, 5,989,913, and 6,642,019 each disclose improvements of microgravity bioreactors.
U.S. Pat. No. 5,576,211 describes a cell culture vessel comprising a cell culture chamber and a nutrient supply chamber separated by a membrane. Further, the cell culture vessel comprises a silicone film for exchange of oxygen and carbon dioxide in the vessel. The cell culture vessel is adapted for rotation.
However, none of these patents or published applications addresses the solution to maintaining high humidity, high gas exchange rates and low water loss from small bioreactors.
Accordingly, it is advantageous to provide an improved microgravity bioreactor that addresses this.